Refactor common code out of globaltrace_trackcallers() and

Refactor common code out of globaltrace_trackcallers() and
globaltrace_countfuncs() into file_module_function_of().

In that function use Michael Hudson's suggestion of gc.get_referrers() to
back up from the code object to a function, then to a class's dict and
finally to a class object if one exists.